Method for producing acrylic synthetic fibers

ABSTRACT

Flexing abrasion resistance and anti-fibrillation of acrylic synthetic fibers are improved by wet-spinning a solution of acrylonitrile polymer in an inorganic solvent through spinnerette orifices while maintaining the linear velocity ratio of the freeextrusion above 1 and the jet-stretch ratio above 1.5.

United States Patent 1 Sato [ METHOD FOR PRODUCING ACRYLIC SYNTHETIC FIBERS [75] Inventor: Mitsunori Sato, Okayama, Japan [73] Assignee: Japan Exlan Company Limited, Osaka, Japan [22] Filed: Mar. 19, 1973 [21] Appl. No.: 342,430

[30] Foreign Application Priority Data Mar. 21, 1972 Japan 47-28296 [52] US. Cl. 264/182; 264/210 F [51] Int. Cl. D011 7/00 [58] Field of Search 264/180-182,

[56] References Cited UNITED STATES PATENTS 2,094,099 9/1937 Dreyfus 264/181 [111 3,885,013 1 1 May 20, 1975 2,975,024 3/1961 Bennett 264/130 3,073,669 l/I963 Fujisaki et a1. 264/182 3,412,191 11/1968 Kitajima et al. 264/182 3,673,053 6/1972 Schimoda et a1..... 264/l 77 Z 3,676,540 7/1972 Story et a1. 264/182 3,689,621 9/1972 Fujii et al 264/182 3,706,828 12/1972 Tzentis 264/182 3,760,053 9/1973 Maranci 264/182 Primary Examiner-Jay H. Woo Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [57] ABSTRACT Flexing abrasion resistance and anti-fibrillation of acrylic synthetic fibers are improved by wet-spinning a solution of acrylonitrile polymer in an inorganic solvent through spinnerette orifices while maintaining the linear velocity ratio of the free-extrusion above 1 and the jet-stretch ratio above 1.5.

2 Claims, 2 Drawing Figures METHOD FOR PRODUCING ACRYLIC SYNTHETIC FIBERS The present invention relates to a method of producing a new acrylic synthetic fiber. More particularly, the invention is concerned with a method of producing a new acrylic synthetic fiber improved in anti-fibrillation and anti-abrasion.

It is well known that acrylic synthetic fibers have wool-like soft touch and good dyeability and are widely used in the field of textile materials as well as interior decoration materials.

However, acrylic synthetic fibers of such great usefulness are not entirely free from practical defects, so that in several technical fields rapid establishment of industrial means has been demanded which will enable the improvement of fiber qualities.

Although the anti-abrasion and anti-fibrillation of acrylic synthetic fibers have reached the practical level required for textile materials almost satisfactorily, these qualities are not necessarily at a level comparable to those of polyester or polyamide synthetic fibers. Thus, the spinning step and various processing steps, e.g. the weaving step, etc. as well as the final products of acrylic synthetic fibers have been accompanied by serious limitations in practical use. More concretely, the insufficiency in knot strength and elongation of the fibers causes not only troubles such as fiber impairment and yarn breaking in the Turbo processing step but also lowers the anti-fibrillation of the final products to damage their commodity value. To date, the solution of such defects still remains pending. Also, the insufficiency in anti-fibrillation and anti-abrasion destroys not only the durability of the final products but also brings about dull color changes of the dyed final products, for example. Thus, the finding of an industrial technical means to obviate such defects caused in the conventional art has been a long standing question of the industry.

It is true that, in response to the demand for modifying the qualities of acrylic synthetic fibers, many industrial means have been proposed in the past. For example, there have been proposed various methods such as the selection of composition of fiber-forming polymer in the production of acrylic synthetic fibers, the formation of a spinning solution by mixing different polymers, the formation of crosslinking structures in the extruded filaments of acrylic synthetic fibers. Another method is for example the regulation of the coagulation as well as the conditions of stretching and heat treatment of the resulting filaments.

However, those methods not only generally make the process complicated but also the regulation of the process condition itself is difficult. Therefore, there are many difficulties in carrying out these methods on an industrial scale.

Thus, we carried on a study to remove the foregoing pending limitations completely in the conventional acrylic synthetic fibers or in the process of the production, and to produce an acrylic synthetic fiber having remarkably improved anti-fibrillation and antiabrasion. As a result, we have found that the objects of the present invention can be effectively attained by maintaining the jet-stretch ratio as well as the linear velocity ratio of the free-extrusion (hereinafter defined) of the filaments in a swollen gel state extruded through spinnerette orifices in a specified range. The present invention is based on this finding.

One of the main objects of the present invention is to provide a new technical knowledge which will improve the anti-fibrillation as well as the anit-abrasion of acrylic synthetic fibers.

Another main object of the present invention is to provide a method of producing a new acrylic synthetic fiber whose color and luster characteristics of the dyed final products are at a level satisfactory enough for practical use. A

A further object of the present invention is to establish a production means for acrylic synthetic fibers which is at a level of productivity advantageous to industrial scale practice, while improving the antiabrasion and anti-fibrillation of acrylic synthetic fibers, as mentioned above.

Other objects of the present invention will become apparent from the following descriptions.

These objects of the present invention can be effectively attained by using a new spinning process wherein a spinning solution of an acrylonitrile polymer in an inorganic solvent is wet-spun through spinnerette orifices, while the linear velocity ratio of the freeextrusion is maintained above 1 and the jet-stretch ratio above 1.5.

The invention will be explained in detail as follows by referring partly to the accompanying drawings wherein,

FIG. 1 is a schematic view of a horizontal type coagulation bath to measure the linear velocity ratio of the free-extrusion, and

FIG. 2 is a graph illustrating the relationship between the linear velocity ratio of the free-extrusion and coagulation bath composition.

Prior to going into detailed explanations of the method of the present invention, some explanations concerning the linear velocity ratio of the freeextrusion defined in the present invention are given in the following.

ln known wet-spinning processes, the coagulation medium for the spinning solution (dope) extruded through spinnerette orifices is a liquid. Thus, to facilitate the circulation of the stream in the coagulation bath and the take-up of the coagulated filaments, it has been common practice to use a horizontal type coagulation bath as shown in FIG. 1. The spinning solution (dope) extruded through the spinnerette orifices travels nearly horizontal through the coagulation bath, while being removed from the solvent, the the resulting filaments are withdrawn from the coagulation bath.

in the coagulation step of spinning solution (dope) by such wet-spinning process, we observed the behavior of fiber formation by varying the takeup speed of the coagulated filaments, while maintaining the extrusion linear velocity of the spinning solution (dope) through the spinnerette orifices constant. As a result, we found that the desolvation behavior of the swollen gel filaments traveling through the coagulation bath can be quantitatively determined as a variation of the takeup tension of the swollen gel filaments, and that the takeup tension is predominantly affected by the concentration of the fiber-forming ploymer in the spinning solution to form the coagulated filaments and by the composition of the coagulation bath.

More particularly, when only the takeup speed of the coagulated filaments is increased, with the linear velocity of the spinning solution (dope) through the spinnerette orifices maintained constant, the takeup tension of the coagulated filaments will gradually increase to finally break the filaments. On the contrary, as the takeup speed is decreased, the tension of the coagulated filaments gradually decrease to reach a relaxed condition substantially free from the influence of external force except the weight itself of the coagulated filaments. Such variation of the takeup tension of the coagulated filaments is influenced by the desolvation behavior of the spinning solution (dope) extruded into the coagulation bath through the spinnerette orifices. However, essentially, the desolvation behavior is more greatly influenced by the concentration of the fiberforming polymer in the spinning solution as well as by the composition of the coagulation bath. In the former case, that is, in the behavior of breaking the coagulated filaments under tension, the takeup speed upon breaking of the coagulated filaments is usually called maximum takeup speed, and the quotient obtained by dividing the maximum takeup speed by an extrusion linear velocity of the spinning solution.(dope) from the spinnerette orifices is defined as maximum jet-stretch ratio, which is used as a physical quantity to evaluate the spinnability. However, usually in the industrial scale practice, spinnerettes having a large number of orifices are used. Thus, it is rather unusual that the coagulated filaments will be broken uniformly at one time as a whole filament bundle by increased takeup tension. Accordingly, it is quite impractical to use the maximum jet-stretch ratio measured on a single coagulated filament as a physical quantity expressing the spinnability or filament-forming characteristics of the extruded fiber bundle. Such tendency of coagulation behavior is particularly noticed when the coagulation ability of the coagulation bath is small for the extruded spinning solution, that is, when the coagulation rate of the swollen gel filaments in the coagulation bath is slow. Also, we made a detailed study on the latter case, that is, on the desolvation behavior and filament-forming behavior of the extruded filaments in a relaxed condition in which the takeup speed of the coagulated filaments is decreased to reduce the takeup tension of the filaments. As a result, we noticed that the takeup speed in a condition that the coagulated filaments are given the lowest possible takeup tension sufficient to maintain a tensioned state between the spinnerette orifices and the drawing rollers has a special significance as a physical quantity expressing the free extrusion state of the spinning solution (dope) in which there is no practical influence of external force on the coagulated filaments except the weight itself of the filaments. We call such extrusion velocity of the spinning solution the linear velocity of free-extrusion, and the quotient obtained by dividing free extrusion velocity by an extrusion velocity of the spinning solution (dope) through the spinnerette orifices is defined as the velocity ratio of the freeextrusion. By the use of the linear velocity ratio of the free-extrusion in combination with the foregoing jetstretch ratio, it has become possible to solve the formation as well as desolvation behavior of the coagulated filaments in the coagulation step more clearly.

In more detailed explanation, the linear velocity ratio of the free-extrusion is not only useful as a practical measure for evaluating the spinnability but has a physicochemical significance as a measure for quantitatively expressing the volumetric diminution rate due to desolvation of the swollen gel filaments in the coagulation bath. Namely, in the case of a large desolvation rate, the volumetric diminution tendency of the extruded swollen gel filaments is large, so that the linear velocity ratio of the free-extrusion becomes reduced. On the contrary, when the desolvation rate is small, the volumetric diminution rate of the swollen gel filaments in the coagulation step becomes reduced to increase the linear velocity ratio of the free-extrusion.

By measuring the linear velocity ratio of the freeextrusion while varying the concentration of the fiberforming polymer contained in the spinning solution as well as the composition of the coagulated bath, we quantitatively determined the desolvation behavior and filament formation behavior, the determination of which is effective for the improvement of anti-abrasion of the fiber. Thus, we attained the present invention.

In more concrete explanation, while varying the concentration of the fiber-forming polymer in the spinning solution and the composition of the coagulation bath, the coagulated filaments are made to travel in a straight line between the spinnerette orifices and the drawing rollers under the takeup tension, as shown in FIG. 1. In such a condition, the exit point C of the coagulated filaments from the coagulation bath surface into an inert medium such as air. Then, the takeup speed is gradually reduced so that the coagulated filaments are sus pended in a relaxed condition between the spinnerette orifices and the drawing rollers, to observe the movement of the point C. Namely, with the decrease of the takeup tension, the exit point C of the coagulated filaments from the coagulation bath surface moves gradually toward the drawing roller side. The coagulated filaments are then held straight in a tensioned condition between the spinnerette orifices and the drawing rollers. Then, while reducing the takeup speed at the moment when a movement of the point C takes place, that is, the takeup speed at the very moment of moving from a tensioned state to a relaxed state. This takeup speed is taken as the linear velocity of the free-extrusion.

In the production of acrylic synthetic fibers, FIG. 2 shows an example of the relation between the linear velocity ratio of the free-extrusion and coagulation bath composition, anti-fibrillation observed with varying concentrations of the fiber-forming polymer in the spinning solution, in the case of using a horizontal coagulation bath in which the immersion length of the coagulated filaments is 300 mm. and the termperature is maintained at -3C.

As a result of repeating a series of systematic experiments on the basis of the above mentioned knowledge, we have found that the flexing abrasion resistance and anti-firbillation of acrylic synthetic fibers can be markedly improved by wetspinning a spinning solution of an acrylonitrile polymer dissolved in an inorganic solvent through spinnerette orifices, while maintaining the linear velocity ratio of the free-extrusion above 1 and the jet-stretch ratio above 1.5.

In carrying out the present invention, to maintain the linear velocity ratio of the free-extrusion in a region above 1, the concentration of the inorganic solvent used in the coagulation bath is desirably adjusted to the range of 40 to percent of the concentration of the inorganic solvent used for dissolving the acrylonitrile polymer in preparing the spinning solution.

Desirably, such concentration of the inorganic solvent used in preparing the spinning solution in the present invention is in the range of 40 to 70 percent.

in the case that the composition of the coagulation bath is outside the foregoing preferred range, it is practically impossible for the spinning solution extruded through the spinnerette orifices to be maintained at a linear velocity ratio of the free-extrusion above 1 and also at a jet stretch ratio above 1.5, the maintaining of this ratio being effective for improving the antiabrasion and anti-fibrillation, together with the maintaining of the foregoing linear velocity ratio of the freeextrusion.

ln the region in which the jet stretch ratio is less than l.5, the gel filaments extruded into the coagulation bath become excessively sagged and consequently wind around the drawing rollers on being drawn from the coagulation bath, the spinnability thus being seriously lowered. Furthermore, in the case of using a jet stretch ratio of less than l.5, however the process conditions in the subsequent heat stretch step, drying step and heat-relaxation step may be regulated, the uniformity of the cross section of the fiber or the high speed spinnability is impaired, and the luster characteristics of the final products, i.e. a secondary effect of the present invention, are greatly lowered. Accordingly, in order to improve the knot strength and elongation and to give acrylic synthetic fiber an arbitrary cross section closely resembling that of the spinnerette orifices and good luster due to the improved smoothness of the fiber surfaces, it is essential to provide a linear velocity ratio of the free-extrusion above I and a jet-stretch ratio above 1.5 for the spinning solution at the same time in the coagulation step.

As another embodiment of the present invention, the following two-stage coagulation process can be carried out without departing from the invention so far as the first bath can staisfy the foregoing preferred range of the linear velocity ratio of free-extrusion and jet-stretch ratio.

Therefore, the following multistage coagulation process to form filaments, for example, may be of course used as an embodiment of the present invention. Namely, after the first-stage coagulation step satisfying the above mentioned linear velocity ratio of the freeextrusion and jet-stretch ratio, the coagulated filaments are further introduced into a second-stage coagulation bath having a solvent concentration of 20 to 30 percent based on the concentration of the inorganic solvent used for the preparation of the spinning solution.

The term acrylic synthetic fibers as referred to in the present invention is a generic term for the fibers composed of an acrylonitrile polymer containing at least 80 percent by weight of combined acrylonitrile. Representative compounds which may copolymerize with acrylonitrile to form acrylonitrile polymers useful for the practice of the present invention are compounds containing a single CH =C group, for instance the vinyl esters, especially the vinyl esters of saturated aliphatic monocarboxylic acids, e.g. vinyl acetate, vinyl propionate, vinyl butyrate, etc.; vinyl halides and vinylidene halides, e.g. vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene chloride, vinylidene bromide, vinylidene fluoride, etc.', allyltype alcohols e.g. allyl alcohol, methallyl alcohol, ethallyl alcohol, etc.; allyl, methallyl and other unsaturated monohydric alcohol esters of monobasic acids, e.g. allyl and methallyl acetates, laurates, cyanides, etc,; acrylic and alkacrylic acids (e.g. methacrylic, ethacrylic, etc.) and esters and amides of such acids (e.g. methyl, ethyl, propyl, butyl, etc. acrylates and methacrylates, acrylamide, methacrylamide, N-methyl, -ethyl, -propyl, butyl,

etc. acrylamides and methacrylamides, etc.); methacrylonitrile, ethacrylonitrile, and other hydrocarbonsubstituted acrylonitriles; unsaturated sulfonic acids containing a single CH =C group and their salts, e.g. allylsulfonic acid, methallylsulfonic acid, styrene sulfonic acid, and their sodium and potassium salts; unsaturated aliphatic hydrocarbons containing a single CH,=C group, e.g. isobutylene; and numerous other vinyl, acrylic and other compounds containing a single CH =C group which are copolymerizable with acrylonitrile to yield thermoplastic copolymers. Alkyl esters of alpha, beta-unsaturated polycarboxylic acids may also be copolymerized with acrylonitrile to form copolymers, e.g. dimethyl, -ethyl, -propyl, -butyl, etc. esters of maleic, fumaric, citraconic, etc. acids.

Among the inorganic solvents which may be used in the present invention may be mentioned; rhodanides e.g. sodium rhodanides, potassium rhodanide, ammonium rhodanide and calcium rhodanide and mixtures of these rhodanides; concentrated aqueous solutions of inorganic salts, e.g. zinc chloride, lithium chloride, etc.; and concentrated aqueous solutions of inorganic acids, e.g. sulfuric acid, nitric acid, etc.

As apparent from the foregoing explanation, the co agulation bath may be the same aqueous inorganic solvent solution as used for the preparation of the spinning solution, although in a concentration of 40 to percent with respect to the concentration of the inorganic solvent in the spinning solution.

The coagulated filaments withdrawn out of the coagulation bath thereafter pass through water-washing, stretching, drying and heat-relaxing treatment, or further followed by a secondary stretching under dry heat, etc. to be formed into an acrylic synthetic fiber im proved in anti-fibrillation and anit-abrasion, to which improvement the present invention is directed.

in such performance of the present invention, strict limitations as in the coagulation step are not necessary for the process requirements in the steps after waterwashing. However, to make the action and effect of the present invention more remarkable, use of any of the following steps in combination with the foregoing steps is recommended as a preferred embodiment of the present invention.

For example, in practicing after structure elimination under specified temperature-humidity conditions mentioned in Japanese Patent Publication No. 8476/ 7, as a drying procedure for the acrylic synthetic fibers formed according to the present invention, the acrylic synthetic fibers improved in surface smoothness by the use of the peculiar coagulation bath of the present invention become more condensed in structure, and furthermore become remarkably improved not only in anti-fibrillation and anti-abrasion but also in optical characteristics, e.g. luster, etc.

In another preferred embodiment of this invention, the knot strength and elongation characteristics of the final product may be remarkably improved by supplying the acrylic synthetic fiber of the present invention after the heatrelaxing treatment to a Turbo Stapler or the like and subjecting it to a secondary stretching of l.05 to 1.60 times in a dry heat atmosphere at I00 to 200C.

For the sake of convenience, the explanation of a linear velocity ratio of the free-extrusion has been heretofore made on the filament formation using a horizontal type coagulation bath. However, once the inorganic solvent compositions of the spinning solution and coagulation bath which are able to maintain the linear velocity ratio of the free-extrusion above 1 are obtained, even if any coagulation bath other than a horizontal type bath may be used, it is possible to attain the action and effect of the present invention effectively.

Thus, by the use of the above-mentioned coagulation step, the method of the present invention can remarkably improve the anit-fibrillation and anti-abrasion of the fibers and, in its industrial practice, greatly contribute to the industry.

The present invention is further explained by examples, but the invention is not limited by the descriptions of the examples.

The parts and percentages in the examples are shown by weight unless otherwise indicated.

EXAMPLE 1 An acrylonitrile copolymer consisting of 91 parts of acrylonitrile, 9 parts of methyl acrylate, and 0.5 part of sodium methallylsulfonate was dissolved in an aqueous solution of sodium rhodanide to prepare a spinning solution representing the polymer concentration as well as the solvent concentration shown in Table l. The spinning solution was then extruded through circular spinnerette orifices into a low temperature coagulation bath of various concentrations of sodium rhodanide, while varying the linear velocity ratio of the freeextrusion and jet-stretch ratio, to form coagulated filaments. Thereafter, the fiber was washed with water and stretched in the usual way, and dried in a humidityconditioned atmosphere of dry bulb temperature of l22C. and wet bulb temperature of 62C. to eliminate the fiber structure, followed by the ordinary heatrelaxing treatment in saturated steam.

The flexing abrasion resistance and degree of fibrillation of the thus obtained acrylic synthetic fiber are also shown in Table 1.

The heat sample represented as Experiment No 1 caused frequent filament breaking even at the jetstretch stage in the coagulation bath because its linear velocity ratio of the free-extrusion was less than 1, so that it became impossible to continue spinning. The Experiment No 2 test sample fiber, whose jet-stretch ratio was out of the recommended range of the present invention, caused excessive sagging during the jet-stretch in. the coagulation bath, and the winding up of the filamerits by drawing rollers became impossible.

On the contrary, the Experiment No 3 and No. 4 test sample fibers which completely satisfy the coagulation bath conditions recommended in the present invention represented remarkably improved anti-abrasion and anti-fibrillation.

In the present invention, as the means for quantitatively expressing anti-abrasion and anti-fibrillation, flexing abrasion resistance and degree of fibrillation are used, whose methods of determination are defined and explained in the followings.

l. Flexing Abrasion Resistance Two single filaments are crossed with each other such that the angle at the crossing point of each filament is 60. The crossed single filaments are moved to rub with each other at the rate of times per minute with the length of the rubbed parts maintained at 10 mm. The number of times required for filament breaking under rubbing load conditions of 0.2 g/d and 0.4 gld is measured on 20 filaments for each load to obtain the averages N, and N From the thus-obtained average rubbing numbers of times required for filament breaking, flexing abrasion resistance is calculated according to the following formula:

Flexing abrasion resistance (log N, log N2)/2 2. Degree of Fibrillation Two-tenth grams of the test sample fiber cut in about 5 mm. lengths is charged together with 300 cc. pure water into a vessel of a stirring apparatus rotating at 17,000 r.p.m. After 2 minutes stirring, the extent of fibrillation is examined under the a microscope of 200 magnifications. The most badly fibrillated filament is given 7 marks and less fibrillated onces are given 6 marks to 1 mark with the decrease of fibrillation. The degree of fibrillation is represented by the total marks of filaments of the sample fiber.

EXAMPLE 2 The same spinning solution as in Example 1 was extruded through rectangular spinnerette orifices into a coagulation bath. Thereafter, the resulting filaments were passed through the same process steps as in Example l to produce and acrylic synthetic fiber. The coagulation bath conditions and the flexing abrasion resistance as well as the degree of fibrillation of the finally obtained sample fiber are shown in Table 2.

COMPARATIVE EXAMPLE I The acrylonitrile copolymer as described in Example 1 was dissolved in an aqueous solution of sodium rhodanide, and an acrylic synthetic fiber was spun in the usual way. For reference, the linear velocity ratio of the free-extrusion and jet-stretch ratio are mentioned in Table 3. From this comparative example, it can be understood that the flexing abrasion resistance and degree of fibrillation of the acrylic synthetic fiber thus obtained are much lower than those of the acrylic synthetic fiber produced according to the method of the present invention.

Table 3 Cross section of spin- Sodium rhodanide nerette conc. orifices Coagulation bath Spinning solution Sodium rhodanide cone. (3%) Experiment number Polymer cone.

1 44 Circular l Rectangular Freeextrusion linear velocity ratio Jet stretch ratio Experiment number Flexing abrasion resistance fibrillation EXAMPLE 3 The same fibers shown as Experiment No. 3, No. 4 and No. 8 in Example I and Comparative Example 1 were subjected to a secondary stretching of 1.36 times in a dry heat atmosphere at l20C. The knot strength and knot elongation of the thus obtained acrylic synthetic fiber were measured. The results are shown in Table 4.

Table 4 Knot strength Experiment Knot elongation number EXAMPLE 4 The sample fibers shown as Experiment No. 5, No. 6, No. 7 and No. 9 in Example 2 and Comparative Example l were subjected to a secondary stretching in the same dry heat atmosphere as in Example 3. The knot strength and elongation of the final fiber are shown in Table 5.

Table 5 Experiment Knot strength Knot elongation number (g/d) 5 2.20 2.3 6 2.39 7.0 7 2. l4 4.0 8 I.l0 L3 Degree of From the data in Tables 4 and 5, it can be understood that the knot strength and knot elongation of the acrylic synthetic fiber produced according to the method of the present invention are remarkably improved in comparison with those of the conventional acrylic fibers.

What is claimed is:

2. The method as claimed in claim 1, wherein the fibers are removed from the coagulation bath and introduced into a second coagulation bath containing a concentration of the rhodanide which is 20-35 percent of the concentration of the rhodanide in the spinning solu tion. 

1. A METHOD OF PRODUCING ACRYLIC SYNTHETIC FIBERS WHICH COMPRISES EXTRUDING A SPINNING SOLUTION OF AN ACRYLONITRILE POLYMER CONTAINING AT LEAST 80 PERCENT BY WEIGHT OF COMBINED ACRYLONITRILE, DISSOLVED IN A RHODANIDE AS SOLVENT TO PRODUCE A SOLVENT CONCENTRATION OF 40-70 PERCENT BY WEIGHT, THROUGH SPINNERETTE ORIFICES INTO A COAGULATION BATH, CONTAINING A CONCENTRATION OF THE RHODANIDE WHICH IS 40-70 PERCENT OF THE CONCENTRATION OF THE RHODANIDE IN THE SPINNING SOLUTION, WHILE MAINTAINING A LINEAR VELOCITY RATIO OF THE FREE-EXTRUSION ABOVE 1 AND A JET-STREACH RATIO ABOVE 1.5.
 2. The method as claimed in claim 1, wherein the fibers are removed from the coagulation bath and introduced into a second coagulation bath containing a concentration of the rhodanide which is 20-35 percent of the concentration of the rhodanide in the spinning solution. 